The term for a unit of short-term memory brought into common usage in 1956 by George Miller.
A chunk is basicaly what your short-term memory can hold seven (plus or minus two) of. These might be digits, words, nonsense syllables, your area code or pi to 10 places. Anything you know as a single unit.
Once you have about seven chunks in your STM you start losing old ones. Chunks will also start to fade after about 18 seconds if they are not rehearsed.